Lowering Biodiesel Cloud Point with Solid Acid Catalysts

3 October 2006

Researchers at Argentina’s Instituto de Investigaciones en Catlisis y Petroquímica (INCAPE) are working to lower the cloud point of biodiesel by using solid acid catalysts in the production process.

The cloud point of a fuel is the temperature at which crystals of paraffin wax—which will plug up filters—first appear. Crystals can be detected by cloudiness of the fuel. Biodiesel cloud point (as well as other properties such as kinematic viscosity and cetane number) are dependent on the composition of the esters, and can range from around -3° C to +11° C—much higher than the cloud point of petroleum diesel.

Blending biodiesel into petroleum diesel raises the overall cloud point of the blend. Furthermore, Ultra Low-Sulfur Diesel (ULSD) already has a higher cloud point that its higher-sulfur predecessors.

Thus, devising ways to lower the cloud point in biodiesel would prove extremely useful in promoting higher blends of the fuel in cold weather operation.

The INCAPE team isomerized soy biodiesel in the liquid phase at temperatures ranging from 125° C to 275° C using solid acid catalysts (SO42--ZrO2 and H-mordenite) to inhibit the formation of crystals. They then analyzed the crystallization by measuring the cloud point.

Reaction temperatures about 200° C resulted in decreasing the cetane number and increasing coke deposits.

While reacting biodiesel at 150°-200° C, a small decrease (4°-6.5° C) of the cloud point was obtained without a meaningful decrease of the cetane index. Best results were obtained with SO42--ZrO2 at 125° C.

Isomerization had both positive and negative effects on fuel properties: it reduced the cloud point but also increased the viscosity and decreased the cetane index.

Comments

Note that this process does not add sulphur back into the fuel, rather the biodiesel is post-processed for winter weather by reactions that are catalyzed by these minerals at the refinery. In case you're curious, here's more information on mordenite:

http://www.galleries.com/minerals/silicate/mordenit/mordenit.htm

In the process, a fraction of the straight-chained esters (high cetane number = easy to ignite but also high cloud point) are converted into isomers with side chain (both parameters lower).

Lowering the cetane number increases combution noise (aka diesel knock) and the risk of combustion bowl wall wetting (which increases engine-out CO, HC and smoke emissions during engine warm-up). Recent European passenger diesel engine designs have tended to favor wider, shallower piston bowls and larger, earlier pre-injections; if the fuel jet takes too long to ignite, the risk of cylinder wall wetting increases, leading to oil dilution in addition to the above effects.

All of these issues are fairly well understood since refineries already produced summer and winter grades of mineral diesel. Nevertheless, manufacturers would still have to put in extra effort to prove for each and every diesel variant of each and every model in their produce line that it still meets emissions regs when operated with this new, winterized biodiesel. They will do this only if it helps them sell more vehicles, achieve a higher profit per vehicle or, if they have to by law.

Note that a cetane number of 49.5 is well above what EPA and even CARB require for No. 2 mineral diesel. That is because the US demand for gasoline is so high that valuable middle distillates have to be hydrocracked to meet demand. The middle cut of what's left after hydrocracking is called LCO (light cycle oil), which contains high concentrations of isomers, cycloparaffins and aromatics. LCO represents a significant component of US mineral diesel. This is largely independent of the sulphur content and makes it that much harder to achieve US emissions standards using diesel engines.

In Europe, both mineral diesel (EN 590) and biodiesel (EN 14214) must have cetane numbers of 51 or higher.

strictly speaking, you are right as pure biodiesel contains only fatty acid methyl esters (FAMEs), not the true paraffins (aka alkanes) found in mineral diesel.

However, both alkanes and FAME molecules feature long straight hydrocarbon chains that are susceptible to aligning in winter weather, forming solid crystals that can clog fuel filters and interfere with fuel pumps. FAMEs are more prone to this behavior because the relatively polar ester groups tend to increase these intermolecular van der Waals forces, effectively promoting the crystallization process.

These are also the reason why biodiesel exhaust fumes tend to feature a slightly higher proportion of ultra-fine (i.e. dangerous) particulate matter: because individual molecules are a little more likely to stick together, the compressed oxygen in the combustion chamber is slightly less able to attack the fuel's carbon-carbon bonds and initiate the complex chain of combustion reactions. In the combustion chambers of a reciprocal engine, time is of the essence - especially at high RPM and/or if the combustion chamber walls are relatively cold.

However, gravimetric PM10 emissions are still lower for B100 than for mineral diesel which contains some cycloparaffins and aromatics. The geometry of these molecules is even less conducive to oxygen attack.

Note that synthetic diesel compounds produced from synthesis gas via the Fischer-Tropsch process contain alkanes and isomers rather than FAMEs. Therefore, BTL is chemically different from biodiesel.